Drive control for engagement and disengagement of axles of a vehicle

ABSTRACT

The invention concerns a drive control for motor and utility vehicles. The speeds of the wheels and the steering angle are measured by means of sensors. Driving axle slip is determined by comparing the average peripheral wheel speeds of the driven and non driven axles and by taking into account the steering geometry. Driving force and finally chassis efficiency are determined by the slip with the assistance of standard characteristic lines. The efficiencies of all-wheel drive and rear-wheel drive are compared. The drive with the higher degree of efficiency is engaged.

BACKGROUND OF THE INVENTION

The invention concerns a drive control for motor vehicles, in particularfor land and forest utility vehicles. Said drive control produces theengagement or disengagement of additional driving axles together withthe permanently driven axles. Vehicles in which additional driving axlescan be engaged are known already, especially in the field of passengercars and commercial vehicles. German OS 35 05 455 shows a drive controlfor automatic engagement and disengagement of driving axles. In additionto other criteria, the momentarily occurring slip is the main criterionfor engaging and disengaging the all-wheel drive. If the slip of thedriven axle exceeds a limit value, the all-wheel drive is engaged. Ifthe slip falls below a limit value, the all-wheel drive is againdisengaged. The cited publication describes, with a series of differentcontrol factors, a complicated drive control. But it is a disadvantagein this drive control that the slip limit values, even though dependingon operating states such as the vehicle speed, are voluntarilydetermined. A slip whose occurrence is preventable is always associatedwith increased wear and fuel consumption. Especially in agriculturalutility vehicles a slip of the wheels is to be prevented as far aspossible, since thereby the arable soil or the grass stigma becomesdamaged. This results in a reduction of produce. To protect the soil andkeep the damage thereof as small as possible, the power dissipated inthe soil must be minimized, that is, the efficiency of the driving trainmust be maximized.

SUMMARY OF THE INVENTION

The problem to be solved by the invention is to provide a drive controlwhich by the engagement and disengagement of additional driving axlesminimizes the loss of power in the power train caused by the slip, thatis, maximizes the efficiency of the chassis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the drive control and associated vehiclechassis structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The drive control 1 determines in motor or utility vehicles having atleast one driven axle 2 and at least one other engageable driving axle 4whether the latter is engaged or disengaged from the power train orremains there. For this purpose the chassis efficiencies are calculatedwith and without engaged driving axle or driving axles and compared witheach other. If the chassis efficiency with engaged driving axle orengaged driving axles is higher than without engaged driving axles, thelatter are engaged otherwise not. The chassis efficiency is defined bythe quotient of driving power to total power. From said statement, undercertain conditions and simplifications, mathematical formulae are drawnup according to a physical model by means of which the numerical valueof the chassis efficiency is calculated. In the calculated formulae forthe chassis efficiency enter parameters such as forces of gravity uponthe axles, the coefficients of driving force and rolling friction, thewheel peripheral speeds, the ratio of the path speeds of the individualwheels determined by cornering, which among other things depends on thesteering angle, and the wheel slip. The chassis command at least twospeed sensors 8 on a steered axle and at least one speed sensor 10 on adriven axle 2. The steering angle can be calculated from the wheel speedratios of the steered angle with no power engaged. Alternatively, thesteering angle can be determined via a steering angle sensor. Bothmethods can be applied optionally or in combination. From the steeringangle is determined the theoretical ratio of the wheel peripheral speedswithout slip of the individual axles relative to each other. The slip onthe driven axle(s) results then from the comparison of the theoreticalratio of the wheel peripheral speed with the actual peripheral speeds ofthe wheel 12 of the driven and non driven axles determined by the speedsensors. When speaking herebelow of the speed of the wheels of an axle,what is meant, when mentioning only one speed per axle, is always theaverage speed of the wheels of said axle. For certain standardbackground conditions, the functional relationships of the coefficientsof driving force and rolling friction of the slip can be deposited incharacteristic lines in an electronic control unit. With the aid of saidcharacteristic lines these coefficients can be determined from the slipvalues found in the above manner. From said coefficients, the steeringangle, the preset or measured axle load distribution and the slip, thechassis efficiency is calculated in the permanently driven axle(s). Byone other mathematical equation and other characteristic lines, thechassis efficiency when driving is calculated, on one hand, by saidpermanently driven axle or permanently driven axles and, on the other,by one or more added engaged driving axles. For the case that all axlesof the motor vehicle are driven, the slip cannot be determined in theabove manner by comparison of the theoretical ratio of the wheelperipheral speeds with the actual peripheral speeds of the wheels of thedriven and non driven axles, since in this case there are no non drivenaxles by which the actual vehicle speed can be determined. The slip tobe expected in all-wheel drive must therefore be determined in adifferent way. The condition that the traction of the motor or utilityvehicle must be equal when driving by the permanently driven axle oraxles and when all-wheel driving establishes the total slip value andthe coefficients of driving force and rolling friction that enter intothe calculation of the efficiency of the chassis. The relationshipbetween slip in all-wheel drive and coefficients of driving force androlling friction in all-wheel drive is here again deposited in thecharacteristic lines. As already mentioned above, the ratio of thecalculated chassis efficiency is used as engagement criterion for theadditional driving axle(s) wherein for positive slip in a quotient ofthe chassis efficiency, on one hand, with engaged driving axle(s) and onthe other without engaged driving axles of more than 1, the engagementcriterion is fulfilled. For negative slip, that is push slip, theengagement criterion is a quotient of the chassis efficiency, on onehand, with engaged driving axle(s) and, on the other, without engageddriving axles of less than 1. This calculation contains a certainmagnitude of error insofar as the soil property expressed in thecharacteristic lines for coefficients of driving force and rollingfriction is assumed to be standard soil property and does not correspondto the actual. But from this analysis of the numerical data in the testit is demonstrated that the quotient of the chassis efficiency to greatextent is independent of the soil property.

In an advantageous development of the invention the rear axle ispermanently driven and the front axle can be engaged as driving axle.The quotient of the chassis efficiency of all-wheel and rear-wheel drive$\frac{\eta_{A}}{\eta_{H}}$

is used as engagement criterion of the all-wheel drive wherein the rearwheel slip

i _(H)≧0 $\frac{\eta_{A}}{\eta_{H}} \geq \quad 0$

applies and for the rear wheel slip

i _(H)≦0 $\frac{\eta_{A}}{\eta_{H}} < 0$

applies.

The chassis efficiency n_(H) for the rear-wheel drive is given by thequotients from the wheel traction power P_(T) of the total chassis andthe wheel hub power P_(N) _(—) _(H) of the driven rear axle,$\eta_{H} = \frac{P_{T}}{P_{N\quad \_ \quad H}}$

wherein the wheel traction power P_(T) is composed of the wheel tractionpower P_(T) _(—) _(H) of the rear axle and the rolling friction losspower P_(R) _(—) _(V) of the front axle:$\eta_{H} = \frac{P_{{T\quad \_ \quad H}\quad} - P_{R\quad \_ \quad V}}{P_{N\quad \_ \quad H}}$

The wheel traction power P_(T) _(—) _(H) of the rear axle is the productfrom the wheel traction F_(T) _(—) _(H) of the rear axle and the speedV_(H) of the rear axle over ground. The rolling friction power lossP_(R) _(—) _(V) of the front axle is the product from the rollingfriction force F_(R) _(—) _(V) of the front axle and the speed v_(V) ofthe front axle over ground. As result of the different road lengthswhich the front axle and the rear axle cover when cornering, said speedv_(V) is different from the speed v_(H) of the rear axle. The hub powerP_(N) _(—) _(H) of the driven rear axle is the product from the wheelperipheral force F_(U) _(—) _(H) and wheel peripheral speed v_(O) _(—)_(H) of the rear axle. The wheel peripheral speed v_(O) _(—) _(H) isdefined by the product from the angular speed of the wheel and radius ofthe wheel. Thus results for the chassis efficiency η_(H) for rear wheeldrive:$\eta_{H} = \frac{{F_{{T\quad \_ \quad H}\quad}v_{H}} - {F_{R\quad \_ \quad V}v_{v}}}{F_{U\quad \_ \quad H}v_{O\quad \_ \quad H}}$

The wheel traction F_(T) _(—) _(H) of the rear axle is associated asproportionality factor via the coefficient of driving force or positiveengagement K _(h) _(—) _(H) in rear wheel drive with the rear axleportion of weight F_(Gh) of the vehicle. The rolling friction forceF_(R) _(—) _(V) of the front axle over the rolling friction coefficientρ_(v) of the front axle in rear-wheel drive is analogously proportionalto the front axle portion of the weight force F_(Gv) of the vehicle. Thewheel peripheral force F_(U) _(—) _(H) is the product of the sum ofdriving force coefficient K _(h) _(—) _(H) in rear-wheel drive androlling friction coefficient ρ_(h) _(—) _(H) of the rear axle inrear-wheel drive and the rear axle portion of the weight F_(Gh) of thevehicle. Thus the chassis efficiency η_(H) for rear-wheel drive can beexpressed as follows:$\eta_{H} = \frac{{\kappa_{h\quad \_ \quad H}F_{Gh}v_{H}} - {\rho_{v}F_{Gv}v_{V}}}{\left( {\kappa_{h\quad \_ \quad H} + \rho_{h\quad \_ \quad H}} \right)F_{Gh}v_{O\quad \_ \quad H}}$

If the wheel peripheral speed v_(O) _(—) _(H) of the rear axle is higherthan the speed v_(H) of the rear axle over ground, then the wheel slipis a pull slip or a positive slip. The average slip i_(H) of the rearwheels of the vehicle in rear-wheel drive is then defined as follows:$i_{H} = {1 - \frac{v_{H}}{v_{O\quad \_ \quad H}}}$

With this definition the speed v_(H) of the rear axle over ground can beexpressed by the average slip i_(H) of the rear wheels and the wheelperipheral speed v_(O) _(—) _(H) of the rear axle as follows:

v _(H)=(1−i _(H))v_(O) _(—) _(H)

The speed v_(V)—as already mentioned above—differs from the speed v_(H)of the rear axle as result of the different road lengths which thewheels of the front axle and those of the rear axle cover whencornering. If the parameter s is introduced as the ratio of the averagepath speeds of the front wheels and rear wheels, the speed v_(V) can beexpressed by the speed v_(H) of the rear axle

v _(V) =sv _(H)

and via the above equation by the slip i_(H) of the rear wheels and thewheel peripheral speed v_(O) _(—) _(H) of the rear axle can be written

v _(V) =S(1−i _(H))v _(O) _(—) _(H)

With the aid of these equations the following formula results for thechassis efficiency in rear-wheel drive for positive slip$\eta_{H} = {\frac{\kappa_{h\_ H} - {s\quad \rho_{v}\frac{F_{Gv}}{F_{Gh}}}}{\kappa_{h\_ H} + \rho_{h\_ H}}*\left( {1 - i_{H}} \right)}$

The mathematical equation for the chassis efficiency is all-wheel drivefor positive slip is derived in analogous manner.

The chassis efficiency η_(A) for all-wheel drive is given by thequotients from the wheel traction force P_(T) of the whole chassis andthe wheel hub power P_(N) _(—) _(A) of the driven axle$\eta_{H} = \frac{P_{T}}{P_{N\quad \_ \quad A}}$

wherein the wheel traction power P_(T) is here composed in case of theall-wheel drive from the wheel traction power P_(T) _(—) _(H) of therear axle and the wheel traction power P_(T) _(—) _(V) of the frontaxle:$\eta_{H} = \frac{P_{T\quad \_ \quad H} + P_{T\quad \_ \quad V}}{P_{N\quad \_ \quad A}}$

The wheel traction power P_(T) _(—) _(H) of the rear axle is the productfrom the wheel traction force F_(T) _(—) _(H) of the rear axle and thespeed v_(H) of the rear axle over ground. The wheel traction power P_(T)_(—) _(V) of the front axle is analogously the product from the wheeltraction force F_(T) _(—) _(V) of the front axle and the speed v_(V) ofthe front axle over ground. Said speed v_(V) is—like in the rear-wheeldrive—different from the speed v_(H) of the rear axle as result of thedifferent road lengths which the wheels of the front axle and those ofthe rear axle cover when cornering. The wheel hub power P_(N) _(—) _(A)of the driven rear and front axles is the sum of the product from wheelperipheral force F_(U) _(—) _(H) and wheel peripheral speed v_(O) _(—)_(H) of the rear axle and of the product from wheel peripheral foreF_(U) _(—) _(V) and wheel peripheral speed v_(O) _(—) _(V) of the frontaxle. Wheel peripheral speeds V_(O) _(—) _(H) and v_(O) _(—) _(V) arerespectively defined by the product from angular speed of the wheel andradius of the wheel.

Thus results for the chassis efficiency ηA for all wheel drive$\eta_{H} = \frac{{F_{T\_ H}v_{H}} + {F_{T\_ V}v_{V}}}{{F_{U\_ H}v_{O\_ H}} + {F_{U\_ V}v_{O\_ V}}}$

The wheel traction force F_(T) _(—) _(H) of the rear axle, via thedriving force or positive engagement coefficient K _(h) _(—) _(A) of therear axle in all-wheel drive, is associated as proportionality factorwith the rear wheel portion of the weight F_(Gh) of the vehicle.Similarly the wheel traction force F_(T) _(—) _(V) of the front axle,via driving force or positive engagement coefficient K _(V) _(—) _(A) ofthe front axle in all-wheel drive, is associated as proportionalityfactor with the front axle portion of the weight F_(GV) of the vehicle.The wheel peripheral force F_(U) _(—) _(H) of the rear axle is theproduct from the sum of driving force coefficient K _(h) _(—) _(A) ofthe rear axle in all-wheel drive and rolling friction coefficient ρ_(h)_(—) _(A) of the rear axle in all-wheel drive and the rear axle portionof the weight F_(Gh) of the vehicle. Similarly to this the wheelperipheral force F_(U) _(—) _(V) of the front axle is the product fromthe sum of driving force coefficient K _(v) _(—) _(A) of the front axlein all-wheel drive and rolling friction coefficient ρ_(v) _(—) _(A) ofthe front axle in all-wheel drive and the front axle portion of theweight F_(Gv) of the vehicle. Thus, the chassis efficiency η_(A) forall-wheel drive can be expressed as follows:$\eta_{A} = \frac{{\kappa_{h\_ A}F_{Gh}v_{H}} + {\kappa_{v\_ A}F_{Gv}v_{V}}}{{\left( {\kappa_{h - A} + \rho_{h\_ A}} \right)F_{Gh}v_{O\_ H}} + {\left( {\kappa_{v - A} + \rho_{v\_ A}} \right)F_{Gv}v_{O\_ V}}}$

If the wheel peripheral speeds v_(O) _(—) _(H) and v_(O) _(—) _(v) ofthe rear and front axles are higher than the speeds v_(H) and v_(V) ofthe rear axle over ground, then the wheel slip is a pull slip orpositive slip. The average slip i_(A) of the rear wheels of the vehiclein all-wheel drive is then defined as follows:$i_{A} = {1 - \frac{v_{H}}{v_{O\_ H}}}$

With this definition the speed v_(H) of the rear axle over ground can beexpressed by the average slip i_(A) of the rear wheels in all-wheeldrive and the wheel peripheral speed v_(O) _(—) _(H) of the rear axle asfollows:

v _(H)=(1−i _(A))v _(O) _(—) _(H)

The speed v_(V)—as already mentioned above—is different from the speedv_(H) of the rear axle as result of the different road lengths which thewheels of the front axle and those of the rear axle cover in cornering.If the parameter s is introduced as ratio of the average path speeds ofthe front wheels and rear wheels, then the speed v_(V) can be expressedby the speed v_(H) of the rear axle:

v _(V) =sv _(H)

By the above equation the speed v_(V) of the front axle over groundthrough the slip i_(A) of the rear wheels in all-wheel drive and thewheel peripheral speed V_(O) _(—) _(H) of the rear axle can be writtenas follows:

v _(V) =s(1=i _(A))v _(O) _(—) _(H)

The wheels of the rear and front axles do not necessarily have to bedriven at the same speed. Specially when cornering it is convenient todrive the wheels of the front axle quicker than those of the rear axle,since they have to corner a wider path. This means that the ratio e ofthe average driving speeds of the front wheels and rear wheels do notalways necessarily have to equal 1. The wheel peripheral speed v_(O)_(—) _(V) of the front axle can thus be expressed by the wheelperipheral speed v_(O) _(—) _(H) of the rear axle:

v _(O) _(—) _(V) =ev _(O) _(—) _(H)

With the aid of these equations there results for the chassis efficiencyin all-wheel drive for positive slip the following formula:$\eta_{A} = {\frac{\kappa_{h\_ A} + {{s\kappa}_{v\_ A}\frac{F_{Gv}}{F_{Gh}}}}{\kappa_{h\_ A} + \rho_{h\_ A} + {{e\left( {\kappa_{h\_ A} + \rho_{h\_ A}} \right)}\frac{F_{Gv}}{F_{Gh}}}}*\left( {1 - i_{A}} \right)}$

If the wheel peripheral speeds v_(O) _(—) _(H) and v_(O) _(—) _(V) ofthe rear and front axles are lower than the speeds v_(H) and v_(V) ofthe rear axle over ground, then the wheel slip is a push slip ornegative slip. The average slip i_(H) of the rear wheels of the vehiclein the rear-wheel drive is in this case defined as follows:$i_{H} = {\frac{v_{O\_ H}}{v_{H}} - 1}$

According to this definition there similarly applies to the average slipi_(A) of the rear wheels of the vehicle in all-wheel drive:$i_{A} = {\frac{v_{O\_ H}}{v_{H}} - 1}$

For negative slip the above equation for the chassis efficiency inrear-wheel drive thus change as follows:$\eta_{H} = {\frac{\kappa_{h\_ H} + {{s\rho}_{v}\frac{F_{Gv}}{F_{Gh}}}}{\kappa_{h\_ H} + \rho_{h\_ H}}*\frac{1}{1 + i_{H}}}$

For the chassis efficiency in all-wheel drive to the negative slipapplies the equation:$\eta_{A} = {\frac{\kappa_{h\_ A} + {{s\kappa}_{v\_ A}\frac{F_{Gv}}{F_{Gh}}}}{\kappa_{h\_ A} + \rho_{h\_ A} + {{e\left( {\kappa_{h\_ A} + \rho_{h\_ A}} \right)}\frac{F_{Gv}}{F_{Gh}}}}*\frac{1}{1 + i_{A}}}$

As already mentioned above, the symbols designate:

η_(H) chassis efficiency in rear-wheel drive

η_(A) chassis efficiency in all-wheel drive

K _(h) _(—) _(H) coefficient of driving force in rear-wheel drive

K _(h) _(—) _(A) coefficient of driving force or rear axle in all-wheeldrive

K _(v) _(—) _(A) coefficient of driving force of front axle in all-wheeldrive

S ratio of the average path speeds of the front wheels and rear wheels

ρ_(v) coefficient of rolling friction of the front axle in rear-wheeldrive

ρ_(h) _(—) _(H) coefficient of rolling friction of the rear axle inrear-wheel drive

ρ_(h) _(—) _(A) coefficient of rolling friction of the rear axle inall-wheel drive

F_(Gv) front axle portion of the weight on the vehicle

F_(Gh) rear axle portion of the weight of the vehicle

i_(H) average slip of the rear wheels of the vehicle in rear-wheel drive

i_(A) average slip of the rear wheels of the vehicle in all-wheel drive

e ratio of the average driving speeds of the front wheels and rearwheels.

The ratio s of the average path speeds of the front wheels and rearwheels can be determined by the equation$s = \frac{1}{\cos {\langle\delta_{v}\rangle}}$

with (δ_(v)) the average steering angle of the wheel of the front axle.

The average slip Of i_(H) of the rear wheels which enters into theformula for the chassis efficiency η_(H) is measured by comparing thetheoretical ratio of the wheel peripheral speeds of front to rear axles,which ratio takes into consideration the steering angle, with the actualperipheral speed ratio of the wheels of the front to rear axles, whichratio is determined by the speed sensors. At the same time with the nondriven front wheels is measured the speed of the front wheels overground and by the steering angle is calculated the speed of the rearwheels over ground. In all-wheel drive it is not possible to determinethe slip in the above manner by comparing the theoretical ratio of thewheel peripheral speeds with the actual peripheral speed ratio of thewheels of the rear axle and front axle, since in this case the frontwheels do not move along undriven and thus cannot measure the speed overground. The slip to be expected when the all-wheel drive is engaged musttherefore be determined in a different way. The condition that thetraction of the motor or utility vehicle must equal in rear-wheel driveand in all-wheel drive, establishes the slip value and the coefficientsof driving force and rolling friction for all-wheel drive which enterinto the calculation of the efficiency of the chassis in all-wheeldrive. As already mentioned above, for certain standard backgroundconditions, the functional relationships of the coefficients of drivingforce and rolling friction of the slip can be deposited incharacteristic lines in an electronic control unit. This applies torear-wheel and to all-wheel drives. By the mathematical equation${\kappa_{h\_ A} + {\kappa_{v\_ A}\frac{F_{Gv}}{F_{Gh}}}} = {\kappa_{h\_ H} - {{s\rho}_{v}\frac{F_{Gv}}{F_{Gh}}}}$

corresponding to the requirement “equal traction in rear-wheel and inall-wheel drives” the slip value, the coefficients of driving force androlling friction for all-wheel drive are determined. Since thefunctional relation of the coefficients of driving force and rollingfriction are certainly preset in characteristic lines under assumptionof standard conditions, the above equation has only a single variable,the slip value in all-wheel drive, and can be solved.

As engagement criterion of the all-wheel drive is used the quotient$\frac{\eta_{A}}{\eta_{H}}$

For the pull slip, that is, for positive slip, to

i _(H)≧0

applies $\frac{\eta_{A}}{\eta_{H}} \geq 1$

and for push slip, that is, for negative slip, to

i _(H)≦0

applies $\frac{\eta_{A}}{\eta_{H}} < 1$

The engagement criterion in the latter case serves to release theservice brake.

As already stated above this calculation contains a certain errormagnitude insofar as the soil property which expresses itself in thecharacteristic lines for coefficient of driving force and rollingfriction, is assumed as standard soil property and does not correspondto the actual. But the analysis of the numeric data in the test showsthat the quotient of the chassis efficiency is to a great extentindependent of the soil property.

In an advantageous development of the invention, after one or moreadditional driving axles have been engaged after a definite period oftime, said axle or axles are again disengaged from the drive train inorder to check again whether the engagement criterion has also beenfulfilled. After the disengagement it is possible, by means of the nondriven axles whose wheels are only entrained, to determine again thespeed over ground of these wheels. By the length ratios of the wheeltrailing tracks dependent on the steering angle, the speed over groundof the driven wheels is thus calculated once more. The above describedcycle for testing the engagement criterion is repeated.

The engagement criterion is preferably re-examined when the steeringangle changes by defined minimum amount. If the steering angle changes,therewith changes the ratio of the track lengths of the wheels of thefront and rear axles. Therewith changes also the slip of the individualwheels and thus the coefficients of driving force and rolling friction.Finally this leads to other chassis efficiencies. To make this clear inan example, a farm tractor with traction load moving straight ahead on aflat road will, in all-wheel drive, show a pull slip on all four wheels.If it is steered to cornering, then the front wheels must cover moremileage than the rear wheels. But when the wheels are driven at fixedratio to each other, it is possible that the front wheels be drivenslower than corresponds to their roll off speed on the curved track.Thus there appears on the front wheels a push slip which results indamage to the chassis efficiency, since the rear wheels must applystronger driving force in order to overcome the resistance of the frontwheels. If the front wheels on the contrary are not driven but only runalong freely, the push slip is perhaps less as result of the rollingfriction and so is the driving force of the rear wheels to be applied.Therefore, it is convenient to re-examine whether the chassis efficiencyduring all-wheel drive is till higher than it is during rear-wheeldrive.

In an advantageous development of the invention, in the case of severaldriven axles or all-wheel drive, the average axle driving speeds can beshifted to different ratios to each other. In order again to make thisclear in the above example: a farm tractor with all-wheel drive issteered to cornering. The front wheels must cover a larger distance thanthe rear wheels. The front and rear wheels are first driven to thesteady ratio of the wheel peripheral speeds of 1:1. The front wheels aremore slowly driven than corresponds to their roll off speed in thelonger curved routes. Thus, a push slip appears on the front wheelswhich leads to impairment of the chassis efficiency. If on the contrarythe front wheels are driven quicker than the rear wheels, their longertrack length in cornering is taken into account. If the ratio of thewheel peripheral speeds of the front and rear wheels is higher than thatof the path speeds, then the front wheels move again with positive slip.

The ratio of the average wheel peripheral dimension of the individualaxles is advantageously determined once more by means of calibrations.The wheel periphery directly enters proportionally into determining theperipheral speed of the wheels by the wheel speed measurement. The wheelperiphery depends on tire abrasion, air pressure in the tiers and loadof the tires. The wheel peripheral speed, therefore, can be determinedonly within said limits. The speed over ground is determined by the nondriven wheels assuming that the slip is zero, that is, the push slipcaused by the rolling friction is disregarded. The speed of the vehicleor of the individual wheels, or the average speed of the wheels of anaxle is then calculated taking into consideration the geometry andsteering geometry of the vehicle. The slip is measured and therewith aredetermined the coefficients of driving force and rolling friction, andfinally the chassis efficiency is ascertained—as described above—bycomparison of the peripheral speeds of the driven and non driven axles.But what is important here is only ratio, not the absolute magnitude ofthe speeds. Thus, to determine the chassis efficiency, it also is onlythe ratio, not the absolute magnitude of the wheel peripheraldimensions, that must be known. Said ratio can be determined by means ofthe speed sensors for the case that no slip occurs on the driven, thesame as non driven wheels, which are used for determining the chassisefficiency. Hence, an operating state must be detected where no slipoccurs. The basic idea here is that when slip occurs, it will betemporarily variable. It is not to be expected that a constant slipvalue in the course of a certain period of time be maintained. Thismeans that the speed ratio of the driven and non driven wheels willconstantly vary when slipping. If on the other hand it is constant for acertain period of time, it can be assumed that no slip occurs. The ratioof the average wheel peripheral dimension in straight ahead motion isconveniently calibrated without actuation of the brakes. If under theseconditions the value of said ratio over a pre-defined time interval doesnot change beyond the preset tolerances, that value is stored as newcalibrated value. In another advantageous development of the invention,the ratio of the average wheel peripheral dimensions of the individualaxles is determined by calibrations, the driver indicating the timeinterval in which the calibration conditions are fulfilled. Saidconditions are, similarly as in the above process: only one axle isdriven, the vehicle moves straight ahead and the brakes are notactuated. At the same time the slip of the wheels is again assumed to bezero. Within this time interval is determined the ratio of the averagewheel peripheral dimensions from the mutual ratios of the speeds of theaxles.

The average steering angle of steering axle determined from the steeringangle sensors is preferably tested by the steering angle determined fromthe wheel speed ratio between left and right wheels of said axle. Thesteering angle determined by the wheel speed ratio between left andright wheels is generally the exact value. But again it is a conditionhere that no slip occurs in the steered wheels. The wheels convenientlymust not be driven or braked.

The additional driving axles are preferably not engaged until theengagement criterion remains valid for a predefined minimum time, or thedistance in time between two moments with valid engagement criterion isshorter than a preset minimum time interval. Otherwise in a limit rangein which the chassis efficiency of the rear-wheel and all-wheel drivesis almost equal, there would be continued shifting back and forth whichwould greatly impair the driving comfort.

The additional driving axles are advantageously engaged when the servicebrake is actuated. In this way it is prevented in a vehicle withoutcenter differential that only the non engaged driving axles or only thecontinuously driven driving axles block. The braking action of the axlesis as a rule uniformly distributed.

The additional driving axles are preferably disengaged when a steeringbrake is actuated. This is convenient when the steering axle is driven,since otherwise the input acts against the brake.

In an advantageous development of the invention, the all-wheel drive isengaged only in a speed range below approximately 15 km/h, said rangereaching up to approximately 17 km/h upon accelerations of the vehicleand up to approximately 13 km/h upon decelerations. Otherwise a suddenshift would have negative effects on the driving characteristic. Thislimitation of course should not apply in braking and engage theall-wheel drive.

What is claimed is:
 1. A drive control for a power train of a motorvehicle having a constantly driven axle and an engageable axle, thedrive control comprising: an electronic control unit for receiving datafrom first and second speed sensors positioned on a steerable axle fordetermining a peripheral speed of a pair of wheels supported by thesteerable axle and a third speed sensor positioned on the driven axlefor measuring a peripheral speed of a pair of wheels supported by thedriven axle, wherein the electronic control unit includes means fordetermining engagement and disengagement of the engageable axle and themeans for determining including: means for obtaining from the first, thesecond and the third speed sensors initial data comprising an actualperipheral speed of the wheels of both the driven axle and theengageable axle; means for determining a steering angle from one of aperipheral wheel speed ratio of each of the wheels of the steered axleand a steering angle sensor; means for calculating from the steeringangle a theoretical ratio of a relative peripheral speed of the wheelswithout slip for the driven axle and the engageable axle; means forcomparing the actual peripheral speed of the wheels of the driven axleand the engageable axle obtained from the first, the second and thethird speed sensors with the theoretical ratio of the peripheral wheelspeeds to determine slip of the driven axle; means for generating, inthe electronic control unit, initial characteristic lines from thedetermined slip to obtain a coefficient of driving force and acoefficient of rolling friction; means for calculating a theoreticalchassis efficiency of the driven axle via a first mathematical equationcorresponding to a physical model using the coefficient of drivingforce, the coefficient of rolling friction, the steering angle, the slipand one of a preset axle load distribution and a measured axle loaddistribution; means for obtaining further data via the first sensor, thesecond sensor and the third sensor from the driven axle and theengageable axle; means for generating further characteristic lines todetermine an actual slip value and an actual coefficient of drivingforce and an actual coefficient of rolling friction based upon acondition where a drive traction of the driven axle is equal to a drivetraction of at least one additional driving axle; means for calculatingan actual chassis efficiency via a second mathematical equation and thefurther characteristic lines; means for obtaining a ratio of thecalculated chassis efficiency and the theoretical chassis efficiency, tobe used by the electronic control unit as engagement criterion fordetermining engagement of the engageable axle; and means for engagingthe second engageable axle when the engagement criterion is greater than1 and in the event of positive slip, and, disengaging the engageableaxle when the engagement criterion is less than 1 and in the event ofnegative slip.
 2. The drive control according to claim 1, wherein a rearaxle is the driven axle and a front axle is the engageable axle and, inthe event of positive slip where only the rear axle is driven, means forapplying the following mathematical equation for chassis efficiency:$\eta_{H} = {\frac{\kappa_{h\_ H} - {s\quad \rho_{v}\frac{F_{Gv}}{F_{Gh}}}}{\kappa_{h\_ H} + \quad \rho_{h\_ H}}*\left( {1 - i_{H}} \right)}$

and, in the event of positive slip where the second engageable axle isdriven, for applying the following mathematical equation for the chassisefficiency:$\eta_{A} = {\frac{\kappa_{h\_ A} + {s\quad \kappa_{v\_ A}\frac{F_{Gv}}{F_{Gh}}}}{\kappa_{h\_ A} + \rho_{h\_ A} + {{e\left( {\kappa_{h\_ A} + \rho_{h\_ A}} \right)}\frac{F_{Gv}}{F_{Gh}}}}*\left( {1 - {i\quad}_{A}} \right)}$

and, in the event of negative slip where only the rear axle is driven,for applying the formula:$\eta_{H} = {\frac{\kappa_{h\_ H} - {s\quad \rho_{v}\frac{F_{Gv}}{F_{Gh}}}}{\kappa_{h\_ H} + \quad \rho_{h\_ H}}*\frac{1}{1 + i_{H}}}$

and, in the event when the second engageable axle is driven, forapplying the formula:$\eta_{A} = {\frac{\kappa_{h\_ A} + {s\quad \kappa_{v\_ A}\frac{F_{Gv}}{F_{Gh}}}}{\kappa_{h\_ A} + \rho_{h\_ A} + {{e\left( {\kappa_{h\_ A} + \rho_{h\_ A}} \right)}\frac{F_{Gv}}{F_{Gh}}}}*\frac{1}{1 + {i\quad}_{A}}}$

where the variables are defined as: η_(H) the chassis efficiency forrear-wheel drive; η_(A) the chassis efficiency for all-wheel drive; K_(h) _(—) _(H) the coefficient of driving force for rear-wheel drive; K_(h) _(—) _(A) the coefficient of driving force for the rear axle forall-wheel drive; K _(v) _(—) _(A) the coefficient of driving force forthe front axle for all-wheel drive; s a ratio of average path speeds ofthe front wheels and the rear wheels; ρ_(v) the coefficient of rollingfriction for the front axle for rear-wheel drive; ρ_(h) _(—) _(H) thecoefficient of rolling friction for the rear axle for rear-wheel drive;ρ_(h) _(—) _(A) the coefficient of rolling friction for the rear axlefor all-wheel drive; F_(Gv) a front axle portion of a weight of avehicle; F_(Gh) a rear axle portion of the weight of the vehicle; i_(H)an average slip of the rear wheels of the vehicle for rear-wheel drive;i_(A) an average slip of the rear wheels of the vehicle for all-wheeldrive; e a ratio of the average driving speeds of the front wheels andrear wheels, in which the ratio of the average path speeds of the frontwheels and the rear wheels is determined by the equation$s = \frac{1}{\cos {\langle{\delta \quad v}\rangle}}$

with (δ_(v)) average steering angle of the wheels of the front axle,wherein the average slip i_(H) of the rear wheels is entered into theformula for chassis efficiency η_(H), in all-wheel drive, the averageslip i_(A) of the rear wheels of the vehicle is measured and entered inthe formula for the chassis efficiency η_(A) in all-wheel drive iscalculated based upon the condition where drive traction in the drivenaxle is equal to the drive traction of the additionally engaged drivingaxle by a corresponding mathematical equation${\kappa_{h\_ A} + {\kappa_{v\_ A}\frac{F_{Gv}}{F_{Gh}}}} = {\kappa_{h\_ H} - {s\quad \rho_{v}\frac{F_{Gv}}{F_{Gh}}}}$

and the quotient $\frac{\eta_{A}}{\eta_{H}}$

is used as the engagement criterion for all-wheel drive, and$\begin{matrix}{i_{H} \geq 0} & {\frac{\eta_{A}}{\eta_{H}} \geq 1} & {applies} \\{{and}\quad {to}} & \quad & \quad \\{i_{H} \leq 0} & {\frac{\eta_{A}}{\eta_{H}} < 1} & {{applies}.}\end{matrix}$


3. The drive control according to claim 1, further comprising, afterengagement of the engageable axle for a desired time period anddisengagement of the engageable axle from a drive train, means fortesting again whether the engagement criterion is fulfilled.
 4. Thedrive control according to claim 1, further comprising means for testingthe engagement criterion when the steering angle changes by a minimumdefined amount.
 5. The drive control according to claim 1, furthercomprising means for shifting an average axle driving speed to differentreciprocal ratios upon driving several driven axles and during all-wheeldrive operation.
 6. The drive control according to claim 1, furthercomprising means for determining a ratio of an average wheel peripheraldimension of the respective axles via calibrations whereby only thedriven axle is driven during forward motion without actuation of thebrakes; and for determining reciprocal ratios of the average speeds forindividual axles and in a ratio is constant for a certain time periodwithin a desired tolerance precision, the slip of the wheels is assumedto be zero and under this assumption the ratio of the average wheelperipheral dimensions is determined from the reciprocal ratios of theaverage speeds of the axles.
 7. The drive control according to claim 1,further comprising means for determining a ratio of an averageperipheral wheel dimension for the axles via calibrations and only thedriven axle is driven and the drive control indicates a time interval inwhich, during forward motion without actuation of the brakes, the slipof the wheels can be assumed to be zero and in the time interval theratio of the average wheel peripheral dimensions is determined from thereciprocal ratios of the average speeds of the axles.
 8. The drivecontrol according to claim 1, further comprising means for determiningan average steering angle of a steering axle by a comparison between afirst steering angle measured by the steering angle sensors and a secondsteering angle determined by the wheel speed ratio.
 9. The drive controlaccording to claim 8, further comprising means for determining theaverage steering angle via a non driven steering axle.
 10. The drivecontrol according to claim 1, further comprising means for preventingthe engageable driving axle from engaging unless the engagementcriterion is present for a predefined time interval.
 11. The drivecontrol according to claim 1, further comprising means for engaging theengageable driving axle when the service brake is actuated.
 12. Thedrive control according to claim 1, further comprising means fordisengaging the engageable driving axle when a steering brake isactuated.
 13. The drive control according to claim 1, further comprisingmeans for engaging the driven axle only when a speed value of a vehicleis below approximately 15 km/h, and means for increasing the speed valueto approximately 17 km/h during acceleration of the vehicle anddecreasing the speed value to approximately 13 km/h upon deceleration ofthe vehicle.
 14. A method of controlling a drive control for a powertrain of a motor vehicle having a constantly driven axle and anengageable axle, the drive control comprising an electronic control unitfor receiving data from first and second speed sensors positioned on asteerable axle for determining a peripheral speed of a pair of wheelssupported by the steerable axle and a third speed sensor positioned onthe driven axle for measuring a peripheral speed of a pair of wheelssupported by the driven axle, wherein the electronic control unitdetermines engagement and disengagement of the engageable axle, and themethod comprises the steps of: obtaining from the first, the second andthe third speed sensors initial data comprising an actual peripheralspeed of the wheels of both the driven axle and the engageable axle;determining a steering angle from one of a peripheral wheel speed ratioof each of the wheels of the steered axle and a steering angle sensor;calculating from the steering angle a theoretical ratio of a relativeperipheral speed of the wheels without slip for the driven axle and theengageable axle; comparing the actual peripheral speed of the wheels ofthe driven axle and the engageable axle obtained from the first, thesecond and the third speed sensors with the theoretical ratio of theperipheral wheel speeds to determine slip of the driven axle;generating, in the electronic control unit, initial characteristic linesfrom the determined slip to obtain a coefficient of driving force and acoefficient of rolling friction; calculating a theoretical chassisefficiency of the driven axle via a first mathematical equationcorresponding to a physical model using the coefficient of drivingforce, the coefficient of rolling friction, the steering angle, the slipand one of a preset axle load distribution and a measured axle loaddistribution; obtaining further data via the first sensor, the secondsensor and the third sensor from the driven axle and the engageableaxle; generating further characteristic lines to determine an actualslip value and an actual coefficient of driving force and an actualcoefficient of rolling friction based upon a condition where a drivetraction of the driven axle is equal to a drive traction of at least oneadditional driving axle; calculating an actual chassis efficiency via asecond mathematical equation and the further characteristic lines;obtaining a ratio of the calculated chassis efficiency and thetheoretical chassis efficiency, to be used by the electronic controlunit as engagement criterion for determining engagement of theengageable axle; and engaging the engageable axle when the engagementcriterion is greater than 1 and in the event of positive slip, and,disengaging the engageable axle when the engagement criterion is lessthan 1 in the event of negative slip.
 15. The method according to claim14, wherein a rear axle is driven axle and a front axle is theengageable axle and, in the event of positive slip where only the rearaxle is driven, further comprising the steps of applying the followingmathematical equation for chassis efficiency:$\eta_{H} = {\frac{\kappa_{h\_ H} - {s\quad \rho_{v}\quad \frac{F_{Gv}}{F_{Gh}}}}{{\kappa_{h\_ H} + \rho_{h\_ H}}\quad}*\left( {1 - i_{H}} \right)}$

and, in the event of positive slip where the second engageable axle isdriven, applying the following mathematical equation for the chassisefficiency:$\eta_{A} = {\frac{\kappa_{h\_ A} + {s\quad \kappa_{v\_ A}\frac{F_{Gv}}{F_{Gh}}}}{\kappa_{h\_ A} + \rho_{h\_ A} + {{e\left( {\kappa_{h\_ A} + \rho_{h\_ A}} \right)}\frac{F_{Gv}}{F_{Gh}}}}*\left( {1 - {i\quad}_{A}} \right)}$

and, in the event of negative slip where only the rear axle is driven,applying the formula:$\eta_{H} = {\frac{\kappa_{h\_ H} - {s\quad \rho_{v}\frac{F_{Gv}}{F_{Gh}}}}{\kappa_{h\_ H} + \quad \rho_{h\_ H}}*\frac{1}{1 + i_{H}}}$

and, in the event when the second engageable axle is driven, applyingthe formula:$\eta_{A} = {\frac{\kappa_{h\_ A} + {s\quad \kappa_{v\_ A}\frac{F_{Gv}}{F_{Gh}}}}{\kappa_{h\_ A} + \rho_{h\_ A} + {{e\left( {\kappa_{h\_ A} + \rho_{h\_ A}} \right)}\frac{F_{Gv}}{F_{Gh}}}}*\frac{1}{1 + i_{A}}}$

where the variables are defined as: η_(H) the chassis efficiency forrear-wheel drive; η_(A) the chassis efficiency for all-wheel drive; K_(h) _(—) _(H) the coefficient of driving force for rear-wheel drive; K_(h) _(—) _(A) the coefficient of driving force for the rear axle forall-wheel drive; K _(v) _(—) _(A) the coefficient of driving force forthe front axle for all-wheel drive; s a ratio of average path speeds ofthe front wheels and the rear wheels; ρ_(v) the coefficient of rollingfriction for the front axle for rear-wheel drive; ρ_(h) _(—) _(H) thecoefficient of rolling friction for the rear axle for rear-wheel drive;ρ_(h) _(—) _(A) the coefficient of rolling friction for the rear axlefor all-wheel drive; F_(Gv) a front axle portion of a weight of avehicle; F_(Gh) a rear axle portion of the weight of the vehicle; i_(H)an average slip of the rear wheels of the vehicle for rear-wheel drive;i_(A) an average slip of the rear wheels of the vehicle for all-wheeldrive; e a ratio of the average driving speeds of the front wheels andrear wheels, in which the ratio of the average path speeds of the frontwheels and the rear wheels is determined by the equation:$s = \frac{1}{\cos {\langle\delta_{v}\rangle}}$

with (δ_(v)) average steering angle of the wheels of the front axle,wherein the average slip i_(H) of the rear wheels is entered into theformula for chassis efficiency η_(H) in all-wheel drive, the averageslip i_(A) of the rear wheels of the vehicle is measured and entered inthe formula for the chassis efficiency η_(A) in all-wheel drive iscalculated based upon the condition where drive traction in the drivenaxle is equal to the drive traction of the additionally engaged drivingaxle by a corresponding mathematical equation${{\kappa_{h\_ A} + {\kappa_{v\_ A}\frac{F_{Gv}}{F_{Gh}}}} = {\kappa_{h\_ H} - {s\quad \rho_{v}\frac{F_{Gv}}{F_{Gh}}}}}\quad$

and the quotient $\frac{\eta_{A}}{\eta_{H}}$

is used as the engagement criterion for all-wheel drive, and applying$\frac{\eta_{A}}{\eta_{H}} \geq 1.$


16. The method according to claim 14, further comprising the step of,after engagement of the second engageable axle for a desired time periodand disengagement of the engageable axle from a drive train, testingagain whether the engagement criterion is fulfilled.
 17. The methodaccording to claim 14, further comprising the step of testing theengagement criterion when the steering angle changes by a minimumdefined amount.
 18. The method according to claim 14, further comprisingthe step of shifting an average axle driving speed to differentreciprocal ratios upon driving several driven axles and during all-wheeldrive operation.
 19. The method according to claim 14, furthercomprising the step of determining a ratio of an average wheelperipheral dimension of the respective axles via calibrations wherebyonly the driven axle is driven during forward motion without actuationof the brakes; and for determining reciprocal ratios of the averagespeeds for individual axles and in a ratio is constant for a certaintime period within a desired tolerance precision, the slip of the wheelsis assumed to be zero and under this assumption the ratio of the averagewheel peripheral dimensions is determined from the reciprocal ratios ofthe average speeds of the axles.
 20. The method according to claim 14,further comprising the step of determining a ratio of an averageperipheral wheel dimension for the axles via calibrations and only thedriven axle is driven and the drive control indicates a time interval inwhich, during forward motion without actuation of the brakes, the slipof the wheels can be assumed to be zero and in the time interval theratio of the average wheel peripheral dimensions is determined from thereciprocal ratios of the average speeds of the axles.
 21. The methodaccording to claim 14, further comprising the step of determining anaverage steering angle of a steering axle by a comparison between afirst steering angle measured by the steering angle sensors and a secondsteering angle determined by the wheel speed ratio.
 22. The methodaccording to claim 21, further comprising the step of determining theaverage steering angle via a non driven steering axle.
 23. The methodaccording to claim 14, further comprising the step of preventing theengageable driving axle from engaging unless the engagement criterion ispresent for a predefined time interval.
 24. The method according toclaim 14, further comprising the step of engaging the engageable drivingaxle when the service brake is actuated.
 25. The method according toclaim 14, further comprising the step of disengaging the engageabledriving axle when a steering brake is actuated.
 26. The method accordingto claim 14, further comprising the steps of engaging the driven axleonly when a speed value of a vehicle is below approximately 15 km/h, andincreasing the speed value to approximately 17 km/h during accelerationof the vehicle and decreasing the speed value to approximately 13 km/hupon deceleration of the vehicle.